Despite their minute size, bacterial cells exhibit an extensive degree of dynamic, programmed organization at the subcellular and molecular levels. A plethora of dedicated cellular components localize to and assemble at appropriate positions and times to ensure fitness in ever-changing environmental conditions, including those encountered when intruding into other organisms. The proposed project seeks to clarify how the functional network of a conserved polar factor controls multiple cell envelope-associated activities in the alpha- proteobacterium Sinorhizobium meliloti, which infects the roots of legume plants to form symbiosis. Specifically, the PodJ1 protein (SMc02230) localizes to the newer cell pole and deletion of the podJ1 gene interferes with flagellar motility, exopolysaccharide production, cell division, and resistance against detergents and osmotic stress. Microarray analysis indicated that podJ1 affects the expression of 129 genes, the majority of which correlate with phenotypes observed in the podJ1 mutant. In the absence of PodJ1, the gene whose expression was most severely reduced is tacA (SMc04011), which encodes a conserved transcriptional activator. In addition, suppressor analysis led to the identification of nine factos that interact genetically with podJ1. Null mutations in two of the suppressors (SMc00067 and SMc03872) appear to abolish synthesis of succinoglycan, an exopolysaccharide critical for symbiosis. Another suppressor encodes the conserved histidine kinase PleC (SMc02369). The proposed research will (1) examine the roles of SMc00067 and SMc03872 in succinoglycan production, (2) investigate the regulatory activities of the essential histidine kinase PleC, and () determine the function of the TacA transcriptional activator. How these factors contribute to different aspects of S. meliloti physiology will be assessed using multiple approaches, including genetic and microarray analyses, phenotypic assays, and microscopy. Results from the investigation will help elucidate regulatory circuits that appear to be iterated in the alpha-proteobacteria group, which includes pathogens such as Brucella and Bartonella. The insights generated will provide better understanding of the spatial and temporal coordination that occurs during infection.